drm/i915: Pin the context as we work on it

Since we now allow the intel_context_unpin() to run unserialised, we
risk our operations under the intel_context_lock_pinned() being run as
the context is unpinned (and thus invalidating our state). We can
atomically acquire the pin, testing to see if it is pinned in the
process, thus ensuring that the state remains consistent during the
course of the whole operation.

Fixes: 84135022 ("drm/i915/gt: Drop mutex serialisation between context pin/unpin")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200109085142.871563-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gem/i915_gem_context.c

@@ -1236,12 +1236,14 @@ gen8_modify_rpcs(struct intel_context *ce, struct intel_sseu sseu)
 	 * image, or into the registers directory, does not stick). Pristine
 	 * and idle contexts will be configured on pinning.
 	 */
-	if (!intel_context_is_pinned(ce))
+	if (!intel_context_pin_if_active(ce))
 		return 0;
 
 	rq = intel_engine_create_kernel_request(ce->engine);
-	if (IS_ERR(rq))
-		return PTR_ERR(rq);
+	if (IS_ERR(rq)) {
+		ret = PTR_ERR(rq);
+		goto out_unpin;
+	}
 
 	/* Serialise with the remote context */
 	ret = intel_context_prepare_remote_request(ce, rq);
@@ -1249,6 +1251,8 @@ gen8_modify_rpcs(struct intel_context *ce, struct intel_sseu sseu)
 		ret = gen8_emit_rpcs_config(rq, ce, sseu);
 
 	i915_request_add(rq);
+out_unpin:
+	intel_context_unpin(ce);
 	return ret;
 }
 

drivers/gpu/drm/i915/gt/intel_context.h

@@ -76,9 +76,14 @@ static inline void intel_context_unlock_pinned(struct intel_context *ce)
 
 int __intel_context_do_pin(struct intel_context *ce);
 
+static inline bool intel_context_pin_if_active(struct intel_context *ce)
+{
+	return atomic_inc_not_zero(&ce->pin_count);
+}
+
 static inline int intel_context_pin(struct intel_context *ce)
 {
-	if (likely(atomic_inc_not_zero(&ce->pin_count)))
+	if (likely(intel_context_pin_if_active(ce)))
 		return 0;
 
 	return __intel_context_do_pin(ce);

drivers/gpu/drm/i915/i915_debugfs.c

@@ -321,16 +321,15 @@ static void print_context_stats(struct seq_file *m,
 
 		for_each_gem_engine(ce,
 				    i915_gem_context_lock_engines(ctx), it) {
-			intel_context_lock_pinned(ce);
-			if (intel_context_is_pinned(ce)) {
+			if (intel_context_pin_if_active(ce)) {
 				rcu_read_lock();
 				if (ce->state)
 					per_file_stats(0,
 						       ce->state->obj, &kstats);
 				per_file_stats(0, ce->ring->vma->obj, &kstats);
 				rcu_read_unlock();
+				intel_context_unpin(ce);
 			}
-			intel_context_unlock_pinned(ce);
 		}
 		i915_gem_context_unlock_engines(ctx);
 
@@ -1513,15 +1512,14 @@ static int i915_context_status(struct seq_file *m, void *unused)
 
 		for_each_gem_engine(ce,
 				    i915_gem_context_lock_engines(ctx), it) {
-			intel_context_lock_pinned(ce);
-			if (intel_context_is_pinned(ce)) {
+			if (intel_context_pin_if_active(ce)) {
 				seq_printf(m, "%s: ", ce->engine->name);
 				if (ce->state)
 					describe_obj(m, ce->state->obj);
 				describe_ctx_ring(m, ce->ring);
 				seq_putc(m, '\n');
+				intel_context_unpin(ce);
 			}
-			intel_context_unlock_pinned(ce);
 		}
 		i915_gem_context_unlock_engines(ctx);
 

drivers/gpu/drm/i915/i915_perf.c

@@ -2159,8 +2159,6 @@ static int gen8_modify_context(struct intel_context *ce,
 	struct i915_request *rq;
 	int err;
 
-	lockdep_assert_held(&ce->pin_mutex);
-
 	rq = intel_engine_create_kernel_request(ce->engine);
 	if (IS_ERR(rq))
 		return PTR_ERR(rq);
@@ -2203,17 +2201,14 @@ static int gen8_configure_context(struct i915_gem_context *ctx,
 		if (ce->engine->class != RENDER_CLASS)
 			continue;
 
-		err = intel_context_lock_pinned(ce);
-		if (err)
-			break;
+		/* Otherwise OA settings will be set upon first use */
+		if (!intel_context_pin_if_active(ce))
+			continue;
 
 		flex->value = intel_sseu_make_rpcs(ctx->i915, &ce->sseu);
+		err = gen8_modify_context(ce, flex, count);
 
-		/* Otherwise OA settings will be set upon first use */
-		if (intel_context_is_pinned(ce))
-			err = gen8_modify_context(ce, flex, count);
-
-		intel_context_unlock_pinned(ce);
+		intel_context_unpin(ce);
 		if (err)
 			break;
 	}